The present invention relates to a power supply unit for arc processing which is so arranged that an input power of commercial AC power supply of three phase or single phase is rectified into a DC power, and thereafter, inversely converted into a high frequency AC by an inverter circuit, and the high frequency AC power obtained by the inverse conversion is reconverted into a power supply suitable for arc processing through a transformer and a rectifying circuit.
Conventionally, in the arc processing power supply unit usable commonly for high and low voltage powers of a ratio of approximately 1:2 such as commercial power voltages 200 V and 400 V or 230 V and 460 V, a power supply system directly transforming a commercial power into a predetermined voltage can cope with different power supply voltages by changing over the turn ratio of the transformer. However, in the system which is so arranged that a commercial power supply, after being rectified into DC, is inversely converted into AC of a high frequency through an inverter circuit, thereby to convert the high frequency AC into a desired voltage through a transformer, the system changing over the turn ratio is required to use switching elements and diodes having dielectric strengths sufficient to withstand 400 V (or 430 V) and capacities sufficient to withstand a large current at the time of 200 V (or 230 V) in the inverter circuit and rectifying circuit located in the power supply side thereof, which results in a large-sized and expensive circuit and offsets the effect of down-sizing the transformer and the smoothing circuit for subsequent rectifying circuit obtained by raising a frequency through the inverter circuit.
Furthermore, there is also proposed a system so arranged that with respect to the DC voltage output after rectification, a half-bridge type inverter circuit is constituted by two series-connected capacitors and two switching devices at a higher voltage time thereof, and a full-bridge type inverter circuit is constituted by four switching devices at a lower voltage time thereof so that the voltage applied to each switching device constituting the inverter circuit and the output voltage of the inverter circuit may become equal in both cases.
FIG. 1 is a connection diagram showing the conventional unit employing said system, which includes rectifying diodes 1a, 1b, 2a, 2b, 3a, and 3b for full wave rectifying a three phase AC power supply from input terminals U, V and W, capacitors 4a, 4b for smoothing the outputs of diodes 1a to 3b and dividing the output voltage into two partial voltages, switching devices 5a to 5d connected in a bridge with respective series-connection points thereof being connected to a lower voltage contact point of a change-over switch 6 and a primary winding of an output transformer 7, the change-over switch 6 with its higher voltage contact point being connected to the series connection point of the capacitors 4a, 4b and its common contact point being connected to the primary winding of the output transformer 7, and a load 8 connected to the secondary winding of the transformer 7 such as a rectifying circuit and an arc processing load or a rectifying circuit, a low frequency inverter circuit and an arc processing load.
In the unit of FIG. 1, when the three-phase power supply is of 200 V (or 230 V), the change-over switch 6 is connected to the side (a) and switching devices 5a to 5d operate as inverters connected in a bridge, and the primary winding of the output transformer 7 is connected between the connection point of switching devices 5a, 5c and the connection point of switching elements 5b, 5d. In this case, switching devices 5a and 5b, and switching devices 5c and 5d become pairs, respectively, so that respective switching devices in each pair is simultaneously on-off controlled and each pair is alternately on-off controlled by a control circuit (not shown), with capacitors 4a and 4b operating merely as smoothing capacitors.
On the other hand, when the three-phase power supply is of 400 V (or 460 V), the change-over switch 6 is connected to the side (b). In this case, the primary winding of the output transformer 7 is connected between the connection point of capacitors 4a and 4b and the connection point of switching devices 5a and 5c so as to form a half-bridge circuit. As a result, 1/2 of the rectifying output is applied to the transformer 7, with the same voltage output as in the case of 200 V (230 V) being obtained. In this case, switching devices 5a and 5c are on-off operated alternately, with switching elements 5b and 5d being left off.
In the conventional unit, since the same output voltage is obtained in the inverter circuit even when the voltage of the AC input power supply is changed approximately at a ratio of 1:2, the dielectric strength corresponding to the lower voltage is sufficient for the dielectric strength of the switching device in the inverter circuit, but two switching devices become idle in the case of high voltage input, and diodes 1a to 3b constituting the input side rectifying circuit are required to withstand the input voltage of 400 V and the input current at 200 V input. Furthermore, since the constitution of the inverter circuit is changed over between a full bridge system and a half bridge system by the change-over switch 6, the wiring for the inverter circuit becomes long and complicated, the inductance and floating capacity of the wiring is increased. Therefore, when the inverters are operated at a high frequency, the higher surge voltage generated at on-off of switching devices, and the countermeasure and the capacity of the surge absorption circuit require switching devices of a large capacity.